1. Field of the Invention
The present invention relates to a modified-layer forming method of forming a modified layer inside a substrate by directing a laser beam to the substrate such as a semiconductor wafer or the like.
2. Description of the Related Art
A substrate such as a semiconductor wafer is sectioned by predetermined dividing lines into device areas to be formed with devices thereon. After the devices are formed in the device areas, the substrate is subjected to steping such as polishing. The substrate is cut along the predetermined dividing lines to provide the devices (semiconductor chips). Various methods are adopted as substrate cutting methods, including a method of using a cutting blade, and a method of cutting a wafer by directing a laser beam to the wafer to form slits therein by a thermal transpiration phenomenon called ablation.
One of the substrate cutting methods described above is a method of dividing a substrate by forming a modified layer, a fragile area, inside the substrate and applying external force to it for division (see Japanese Patent Laid-Open No. 2005-95952). According to this method, a laser beam capable of passing through a substrate is focused on the inside of the substrate and directed thereto along predetermined cutting lines to form a modified-layer inside the substrate. Thereafter, external force is applied to the substrate to split it from a portion made more fragile by the laser beam. Thus, the substrate is divided into a large number of individual devices along the predetermined dividing lines.
Incidentally, TEGs, test patterns, and stacked articles on a substrate, which absorb or reflect a laser beam, are present on an optical path irradiated with the laser beam in some cases. If a laser beam is directed along a predetermined dividing line with such a stacked article stacked on the optical path of the laser beam, the laser beam is not applied only to an area covered by the stacked article acting as an obstacle. In such a case, since the laser beam does not reach the inside of the substrate, an area formed with no modified-layer is caused in the substrate after the laser beam irradiation. If external force is applied to the substrate in such a condition, since an area formed with no modified-layer is present inside the substrate, a device subjected to the division may be likely to produce chippings, i.e., cracks or burrs. In addition, a problem may occur in that the substrate is split while largely deviating from a predetermined dividing line.